The present invention relates generally to ceramics and more particularly to silicon nitride structural ceramics.
Pure silicon nitride (Si.sub.3 N.sub.4) is a material with great structural strength and resistance to high temperature. Processing this material is unfortunately difficult.
A widely used technique to make silicon nitride more processable is the addition of a small quantity of certain metal oxides such as magnesium oxide (MgO) or aluminum oxide (Al.sub.2 O.sub.3). Since silicon nitride oxidizes, a minute layer of silicon oxide is found on this material. It is believed that the oxide additives combine with the silicon oxide coating to form bonding silicates which then control the high temperature creep and deformation behavior.
The strength of the ceramic depends much on the strength of the bonding silicates. Creep-resistance of the composition is destroyed when the bonding silicates begin to soften as they approach their melting temperatures of .gtoreq. 1400.degree. C. Investigation has shown that certain impurities interfere with the refractory character of the bonding silicate. Consequently research in increasing the high temperature properties of silicon nitride have generally proceeded along two approaches. With the first approach the effort is on decreasing the impurities in the ingredients and the processing, whereas the second approach seeks to develop additives which would produce a more refractory bonding silicate or other refractory bonding phase. The most promising example of the second approach is the additive, yttrium oxide (Y.sub.2 O.sub.3). The resulting bonding silicate has a melting point of almost 2000.degree. C, a higher density, and a high strength. However, Y.sub.2 O.sub.3 is expensive, and unusual detrimental or destructive oxidation effeects have been found in Si.sub.3 N.sub.4 made with Y.sub.2 O.sub.3.